1. Field of the Invention
The invention relates in general to magnetic cores and core-coil assemblies for electrical inductive apparatus, such as distribution transformers, and more specifically to new and improved methods of constructing such magnetic cores of amorphous metal.
2. Description of the Prior Art
Amorphous metal alloys, such as Allied Metglas Product's 2605SC and 2605S-2, exhibit a relatively low no-load loss when used in the magnetic core of an electrical transformer. Thus, the use of amorphous metal alloys appears to be an attractive alternative to conventional grain oriented electrical steel in the construction of magnetic cores for electrical distribution transformers. Although amorphous metal has a higher initial cost than conventional grain oriented electrial steel, the cost difference may be more than offset over the operating life of a transformer by the savings in energy which otherwise would have to be generated to supply the higher losses.
Amorphous metal alloy, however, cannot simply be substituted for conventional electrical steel in the transformer manufacturing process. Amorphous metals possess characteristics which create manufacturing problems which must be economically solved before production line transformers utilizing amorphous metal cores will be readily available in the market place.
For example, amorphous metal is very thin, having a nominal thickness of about 1 mil. Amorphous metal is also very brittle, especially after stress relief anneal, which anneal is necessary after a core is formed of amorphous metal, because amorphous metals are very stress sensitive. The no-load losses of amorphous metals increase significantly after being wound, or otherwise formed into the shape of a magnetic core suitable for distribution transformers. The low no-load loss characteristic is then restored by the stress-relief anneal.
The thin, brittle amorphous metal strip also makes the forming of the conventional core joint a different manufacturing problem. While the use of a jointless core solves the joint problem, it complicates the electrical windings. Conventional electrical windings, which are simply slipped over the core legs before the conventional core joint is closed, cannot be used with an unjointed core. Techniques are available for winding the high and low voltage windings directly on the legs of an uncut amorphous core, but, in general, these techniques add manufacturing cost and production line complexity.
Another characteristic of amorphous metal cores which creates manufacturing problems is the extreme flexibility of the core after it is wound. For example, a core wound of amorphous metal is not self supporting. When the mandrel upon which the core is wound is removed, the core will collapse from its own weight, if the winding axis is not maintained in a vertical orientation.